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Prerequisites: Quick Start complete, pip install mellea, Ollama running locally. Mellea provides two paths to structured output. Choose based on how the call fits into your code:
PatternWhen to use
@generative with return typeYou want a named, reusable function. The return type is declared in the signature.
instruct(format=...)You are building the prompt dynamically or combining structured output with grounding_context or user_variables.
Both paths enforce the declared schema at generation time using constrained decoding where the backend supports it, and retry with the IVR loop if parsing fails.

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Pattern 1: @generative with typed returns

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Classification with Literal

from typing import Literal
from mellea import generative, start_session

@generative
def classify_priority(issue: str) -> Literal["critical", "high", "medium", "low"]:
    """Classify the priority level of a support issue."""

m = start_session()
priority = classify_priority(m, issue="Production database is unreachable.")
print(priority)
# Output will vary — LLM responses depend on model and temperature.
# Expected: "critical"
The model is constrained to return exactly one of the four allowed values.

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Simple Pydantic extraction

from pydantic import BaseModel
from mellea import generative, start_session

class PersonInfo(BaseModel):
    name: str
    role: str
    department: str

@generative
def extract_person(bio: str) -> PersonInfo:
    """Extract the person's name, role, and department from their biography."""

m = start_session()
bio = "Sarah Chen joined the engineering team in 2021 as a senior backend developer."
person = extract_person(m, bio=bio)
print(person.name, person.role)
# Output will vary — LLM responses depend on model and temperature.

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List returns

Return a list of typed values or Pydantic models: 
from mellea import generative, start_session

@generative
def extract_person_names(doc: str) -> list[str]:
    """Extract the names of all people mentioned in the document."""

m = start_session()
names = extract_person_names(
    m,
    doc="The report was co-authored by Alice Johnson and Bob Lee.",
)
print(names)
# Output will vary — LLM responses depend on model and temperature.
# Expected: ["Alice Johnson", "Bob Lee"]

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Nested models

Complex structured extraction works naturally with nested Pydantic models: 
from pydantic import BaseModel
from mellea import generative, start_session

class Address(BaseModel):
    street: str
    city: str
    country: str

class Company(BaseModel):
    name: str
    industry: str
    headquarters: Address

@generative
def extract_company(text: str) -> Company:
    """Extract company details from the text."""

m = start_session()
company = extract_company(
    m,
    text="Acme Corp is a manufacturing company headquartered at 123 Main St, Springfield, USA.",
)
print(company.headquarters.city)
# Output will vary — LLM responses depend on model and temperature.

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Pattern 2: instruct(format=...)

When you need structured output alongside dynamic prompts, grounding context, or user variables, use the format parameter on instruct(): 
from pydantic import BaseModel
from mellea import start_session
from mellea.stdlib.requirements import check, simple_validate
from mellea.stdlib.sampling import RejectionSamplingStrategy

class NameResponse(BaseModel):
    names: list[str]

m = start_session()
result = m.instruct(
    "Extract ALL person names from the document (doc1).",
    grounding_context={
        "doc1": (
            "Leaders banded together to press Germany to back pro-growth policies. "
            "President Obama gained support for his argument that Europe cannot "
            "afford Chancellor Merkel's austerity approach."
        )
    },
    format=NameResponse,
)

parsed = NameResponse.model_validate_json(str(result))
print(parsed.names)
# Output will vary — LLM responses depend on model and temperature.
# Expected: ["President Obama", "Angela Merkel"]
The format parameter triggers constrained decoding. The result is a ModelOutputThunk whose .value is a JSON string matching the schema. Parse it with PydanticModel.model_validate_json(str(result)).

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Validating structured output content

Constrained decoding enforces schema validity — the output is always parseable JSON matching your model. To enforce semantic constraints (e.g., “the list must contain at least 2 names”), combine format with a custom validation function: 
from collections.abc import Callable
from pydantic import BaseModel, ValidationError
from mellea import start_session
from mellea.stdlib.requirements import check, simple_validate
from mellea.stdlib.sampling import RejectionSamplingStrategy

class NameResponse(BaseModel):
    names: list[str]

def at_least_n_names(n: int) -> Callable[[str], tuple[bool, str]]:
    """Factory: returns a validator that checks the names list has >= n entries."""
    def _validate(text: str) -> tuple[bool, str]:
        try:
            parsed = NameResponse.model_validate_json(text)
        except ValidationError:
            return (False, "Output is not valid JSON matching the NameResponse schema.")
        if len(parsed.names) >= n:
            return (True, "")
        return (False, f"Found {len(parsed.names)} name(s); expected at least {n}.")
    return _validate

m = start_session()
result = m.instruct(
    "Extract ALL person names from the document (doc1).",
    grounding_context={"doc1": "...your document text..."},
    requirements=[
        check(
            None,
            validation_fn=simple_validate(at_least_n_names(2)),
        )
    ],
    strategy=RejectionSamplingStrategy(loop_budget=5),
    format=NameResponse,
    return_sampling_results=True,
)

if result.success:
    names = NameResponse.model_validate_json(str(result.result)).names
    print(names)
else:
    print("Could not extract the required names after retries.")
The check(None, ...) idiom creates a validation-only requirement that is never embedded in the prompt. This avoids biasing the model while still gating the output on your semantic constraint.

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Requirements on @generative output

You can also apply requirements to @generative output. When the return type is a Pydantic model, the requirements operate on the JSON string representation: 
from pydantic import BaseModel
from mellea import generative, start_session
from mellea.stdlib.requirements import req
from mellea.stdlib.sampling import RejectionSamplingStrategy

class Summary(BaseModel):
    title: str
    bullets: list[str]

@generative
def summarize(text: str) -> Summary:
    """Summarize the text as a titled bullet list."""

m = start_session()
summary = summarize(
    m,
    text="...",
    requirements=[req("Include at least 3 bullet points.")],
    strategy=RejectionSamplingStrategy(loop_budget=3),
)
# summary is already a Summary instance — no manual parsing needed
print(summary.title)
for bullet in summary.bullets:
    print(f"  - {bullet}")
# Output will vary — LLM responses depend on model and temperature.
With @generative, the output is parsed into the Pydantic model automatically. You receive a Summary instance, not a JSON string.

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Choosing between the two patterns

Use @generative when:
  • The function is reusable and called from multiple places.
  • The input and output types are stable.
  • You want a clean function signature with IDE type-checking.
  • You prefer direct attribute access (person.name) over manual JSON parsing.
Use instruct(format=...) when:
  • The prompt is built dynamically with user_variables or grounding_context.
  • You are retrofitting structured output onto an existing instruct() call.
  • You need fine-grained control over requirements and sampling alongside formatting.
Both patterns support the full IVR loop, requirements, sampling strategies, and SamplingResult inspection.
See also: Generative Functions | The Requirements System